| Abstract: | In biparental species, parental-investment theory generally predicts an inverse relationship between the level of parental care provided by each parent and incomplete compensation by one parent in response to reduced parental care by their partner. The factors that influence the magnitude of this compensation have rarely been examined in birds. For example, the level of compensation may differ between a widowed bird that receives no assistance from its partner and a mated bird whose partner is still present but providing less than its normal share of parental care. This study compares the compensatory response of female tree swallows (Tachycineta bicolor) without their mate's parental assistance (when females are widowed) and with reduced male parental care (when males are handicapped by cutting some feathers). When compared with control females, experimental females compensated more in terms of nest visits for the absence than the reduction of male parental care. In addition, widowed females had significantly reduced brood mass and fledging success compared with control females. Although handicapped males reduced their nest-visit rate significantly, females with handicapped mates did not significantly increase their nest-visit rate nor was there reduced brood mass or reduced fledging success at their nests. Total nest-visit rate was similar for all groups, yet widowed females fledged fewer and lighter young, suggesting that they brought less food per nest visit. We suggest that fledging success and measures of offspring quality are probably better indicators of the level of compensatory parental care than nest-visit rate. We suggest that for widowed females the benefits of a relatively large compensatory response outweighed the costs; whereas, for females with a handicapped mate the benefits of higher feeding rates were not greater than the cost. The results of this study help to explain the differences among experimental studies of compensatory parental care and point to a new method of testing models of parental care. |
